1. Field of the Invention
The present invention relates to photo-sensor device which per se is known as self-scanning type photo-diode array, CCD photo-sensor, CCD photo-diode array, that is, a combination of CCD and photo diodes and the like. Also, the present invention relates to an image scanning system in which such photo-sensor device is used as image scanning means.
2. Description of the Prior Art
With the rapid and remarkable development of techniques about semiconductor in these years, there have been available various inexpensive self-scanning type photo-sensor devices such as self-scanning type photo-diode array, CCD photo-sensor and CCD photo-diode array, that is, a combination of CCD and photodiodes. This technical innovation also has led us to some attempts to make use of such photo-sensor device as electrical scanning means. For example, attempts have been made to automatically detect the distance to an object or the focusing of optical system to an object by means of scanning output signals derived from such photo-sensor device.
U.S. Pat. No. 4,004,852 has disclosed the use of photo-sensor device in an automatic and electronic range finder instrument. According to the system proposed by the patented invention, a basic view field image of a target object and a reference view field image of the same object are formed by a base line range finder type optical system simultaneously. The reference view field image is so formed as to contain therein the basic view field image and to cover a larger area than that of the latter. These two images are scanned by a photo-sensor device as mentioned above to obtain signals of scanned picture elements of the two images. These signals are binary coded by a binary coding circuit and then stored in shift registers respectively. Data of binary coded picture elements of the basic view field image and those of the reference view field image are compared to each other to know the corelation of the two images. As for the reference view field image, a detection is carried out to find out such image portion which may be considered to be coincident with the basic image or to be most analogous to the latter. From the alignment of such analogous image portion in the reference view field image, namely from the information of position where such analogous image portion is existing, man can know the distance up to the target object.
An electronic type automatic range finder system similar to the above described one is disclosed also in U.S. Pat. No. 4,078,171.
In detecting the distance to an object or the focus of an optical system to an object relying upon the scanning output signals from a photo-sensor device used as image scanning means in a manner as mentioned above, however, some requirements must be satisfied to assure a sufficiently high accuracy of detection. As a matter of course, the circuit for processing the scanning output signals must be improved accordingly. But, the most important problem is to obtain a scanning output signal correctly and precisely corresponding to the image while excluding all noise signals.
As known to those skilled in the art, the scanning output signals read out from a photo-sensor device generally and necessarily contain some disturbing noise signals such as dark current signal which have a great adverse effect on the detection accuracy. In particular, variation of dark current is inherent to such photo-sensor device. When the time required to integrate signals of the photo-sensor device is relatively long, the variation of dark current becomes much increased. Even when the time is relatively short, the dark current may vary greatly with rising up of atmospheric temperature. Therefore, it is absolutely necessary to provide means for effectively excluding noise of dark current from the scanning output signals. Otherwise, no assurance of detection accuracy or no improvement of detection accuracy is obtainable.
Another problem to be solved concerns the variation of voltage applied to the photo-sensor device. In case of CCD photo-sensor or CCD photo-diode array, a photogate voltage must be applied to the light receiving part to have the produced electric charge accumulated therein. On the other hand, to adjust the depth of potential well at the light receiving part, transfer gate part and charge transfer part, it is required to apply a bias voltage to the substrate. As well known in the art, variation in the photo gate voltage or in the substrate bias voltage often results in variation of the level of scanning output signal then obtained. A similar problem occurs also for self-scanning type photo-diode array. In this case, there must be applied a charging voltage to charge the p-n capacitor of each photo diode. Variation in the charging voltage will again lead to variation of the level of scanning output signal then obtained. For the reason, in order to attain a high accuracy of detection as desired, this problem should be solved effectively by providing means for positively excluding such variation component from the scanning output signals.